As regards the heat treatment of liquid or pumpable foods, use may be made of steam in order quickly and efficiently to heat up the food product. One of the methods in existence for introducing steam directly into a product is to employ an injector.
Depending on the temperature to which it is intended to heat the product, it is possible to obtain a pasteurised or an aseptic product, or alternatively a product possessing extended shelf-life in cold storage. Such products may be dairy produce, juices, viscous food products or the like. The commonest method is to heat the product to a temperature where there is a total destruction of harmful micro-organisms. This gives a food product which maintains stability on storage at room temperature. No unbroken refrigeration chain for storage is necessary, which may be an advantage above all in the developing countries.
Direct heating of the product, by introducing steam into the product, gives a rapid and efficient heating. As a result of the rapid method, it is possible to reduce treatment time, which in total gives a reduced thermal effect on the product and a product will be obtained which maintains higher quality, above all as regards flavour.
There is a large number of injectors on the market which all are of similar design and construction, with an inlet for the product which is to be treated and an outlet for the ready-treated product. The injector further displays an inlet for steam which is caused, under high pressure, to mingle, through various arrangements with gaps or ducts, with the product and heat it to the desired temperature. A conventional so-called annular gap injector is described in Swedish Patent Specification SE 367 121.
When it is the intention to treat viscous products, such as partly frozen juice concentrate, jams, ice cream mixes, sauces, creams and the like, it may be necessary to employ a different type of injector. One such injector is described, for example, in Swedish Patent Specification SE 517 823. In this type of injector, steam under high pressure enters into the product through a large number of ducts in the product pipe.
Heating by injection may occasionally give rise to sedimentation in the product, above all milk products. This is because of the combination of thermal effect and the mechanical effect which takes place when the steam bubbles entering into the product implode. Sedimentation in the product is entirely a matter of appearance and does not affect its flavour. In order to obviate such sedimentation, it is possible to homogenise the product under elevated pressure.
The distribution of the steam in the product may also constitute a problem as regards injection heating. Above all, this applies when the intention is to heat-treat viscous products, and then in particular in the employment of annular gap injectors. One method of addressing this problem is to employ and injector of the type which has a multiplicity of steam ducts in the product pipe. However, this type of injector has a shorter operative production time since the steam ducts are easily blocked by so-called fouling, in other words when the heated product burns onto the ducts.
A further problem in heating by means of an injector is that disturbing noise often occurs in the injector. The disturbing noise is so-called cavitation noise which occurs when the steam bubbles fed into the product implode. Today, there are no simple methods of obviating the problem of the high noise level.